1. Field of the Invention
This invention relates to an improved process for etching tungsten silicide. More particularly, this invention relates to a tungsten silicide etch process which is highly selective to photoresist and oxide.
2. Description of the Related Art
The use of tungsten silicide over polysilicon as a gate contact or as an interconnect between circuit elements is becoming more prevalent in VLSI technologies as devices geometries become smaller. Tungsten silicide over polysilicon lowers the resistivity and this lower resistivity allows integrated circuit structure geometries to shrink while maintaining the desired performance improvements needed for advanced integrated circuit structures.
Fluorine-based chemistry such as SF.sub.6 has been used to etch tungsten silicide. However, the tendency to undercut both the tungsten silicide and the underlying polysilicon has been a major drawback of the use of this etching system. The use of a BCl.sub.3 /Cl.sub.2 etch system to etch tungsten silicide has also been proposed in an article by Lum and Bondur entitled "Precision 5000 Etch: Tungsten Silicide Process Capability", published in Applied Materials Etch Bulletin Volume VII, Number 1, Spring 1990. In that article the use of a multi-step process is described wherein BCl.sub.3 /Cl.sub.2 chemistry is used to etch tungsten silicide layer and part of the underlying polysilicon until the entire tungsten silicide film is removed, after which a standard polysilicon etch may be carried out using standard HBr/Cl.sub.2 chemistry.
However, the use of a BCl.sub.3 gas is sometimes not preferred because of the low purity of the gas, which may result in the deposition of particles on the wafer; and because of the low vapor pressure of the gas, which may cause it to condense in the apparatus used to convey the gas to the etch chamber.
It would, therefore, be desirable to provide a tungsten silicide etch system which uses gases which will not deposit particles on the wafer, will not easily condense in the apparatus used to deliver the gas to the etch chamber, and which will have a good selectivity for both photoresist and oxide, while permitting the etching of both tungsten silicide and polysilicon.